Wireless communication systems are widely deployed to provide various types of communication content, such as voice, data, multimedia, and so on. Some wireless communication systems also facilitate position determination. For example, in Satellite Positioning Systems (SPS), an SPS transmitter (for example, a satellite) may continuously broadcast, from a known location, a pseudo-random noise (PRN) code that is modulated using an SPS carrier wave. Particular examples of SPS wireless technologies may include, for example, the Global Positioning System (GPS), the Global Navigation Satellite System (GLONASS), etc.
An SPS receiver may receive the PRN code from one or more SPS transmitters. The broadcast of the PRN code from the SPS transmitter may have a known start time and the PRN code may repeat after a known period length. Accordingly, the mobile device may receive the broadcast and obtain a time-of-flight (TOF) measurement. The SPS receiver first generates a local version of the PRN code having the same symbol sequence and start time as the PRN code broadcast by the SPS transmitter. The SPS receiver then receives the PRN code broadcast by the SPS transmitter and compares the received version of the PRN code to the local version of the PRN code. The SPS receiver may then determine a code-phase of the received PRN code by evaluating the amount of delay between the time that the local PRN code is generated and the time that the received PRN code is received. A larger code-phase indicates a longer TOF and a greater distance from the SPS transmitter to the SPS receiver. The calculated distance from a particular SPS transmitter may be referred to as a pseudorange. Multiple pseudoranges may be used to multilaterate and identify a position of the SPS receiver.
The code-phase measurements described above may be used to sense a position of the SPS receiver with precision on the order of several meters. The precision of the sensed position may be limited by the frequency at which the PRN code is repeated (i.e., broadcast and re-broadcast). However, the SPS receiver can achieve greater precision using carrier-phase measurements in addition to code-phase measurements. The PRN code may be provided on a carrier wave having a significantly higher frequency than the PRN code. Because the frequency of the carrier wave is greater than the frequency of the code cycle, position sensing that is based on carrier-phase measurements may be more precise than position sensing based on code-phase measurements. In particular, the SPS receiver may be able to sense position with precision on the order of several centimeters. However, due to the repeating nature of the carrier wave, it is necessary to resolve an integer ambiguity (IA) before sensing position using carrier-phase measurements. Once the IA is resolved, the carrier phase tracking loops must be continuously locked in order to enable continued resolution of the IA.
As noted above, the PRN code is broadcast continually. Continuous broadcast greatly facilitates locking of the carrier wave and continued resolution of the IA resolution. There have been efforts to use existing wireless architectures to provide positioning services and/or augment the positioning services available using SPS. However, many existing wireless communication standards operate in accordance with a time-division duplexing (TDD) wireless architecture, which makes it difficult to maintain a lock on the carrier wave. This is because in TDD systems, a base station (BS) does not broadcast continuously. For example, certain periods of time may be reserved for downlink (i.e., signaling from the BS to a user equipment (UE)), while the remaining periods of time may be reserved for uplink (i.e., signaling from the UE to the BS). The BS may observe radio silence during uplink in order to limit interference with uplink signaling received from the UE. The BS may be configured to cease broadcast of downlink signaling so often that the IA is not resolvable (or alternatively, not resolved long enough to be useful).
As a result, the conventional carrier-phase measurements that are obtainable in SPS may not be obtainable in other wireless architectures, particularly those that rely on TDD. For example, the forthcoming Fifth Generation (5G) wireless architecture may ultimately utilize TDD. Whatever the advantages of the non-continuous signaling, TDD systems that implement non-continuous signaling are not well-equipped to facilitate high-precision carrier-phase measurements of the kind used in SPS. New techniques are required if TDD signaling is to be adapted for purposes of obtaining carrier-phase measurements.